Photoelectric current and stopping Potential

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Photoelectric current and stopping Potential

Set up this circuit.

Photoelectric current and stopping Potential Diagrams

The photons arriving at the metal plate cause photoelectrons to be emitted. The plate is called the "emitter". Those electrons that cross the gap are collected at the other metal plate - called "the collector". This flow of electrons across the gap sets up an emf between the plates that causes a current to flow around the rest of the circuit. That's a photoelectric cell producing a photoelectric current.

Set up this circuit:

Photoelectric current and stopping Potential Diagrams

The emitter gives out electrons. So we call it a cathode.

If the emf of the power supply is initially zero, the circuit works just like the one above this. As the supply is turned up, the emitter becomes more positive (because it is connected to the positive terminal of the supply).

So electrons leaving it are attracted back towards it. If they leave with enough energy they can overcome this attraction and cross to the collector. If they don't have enough energy, they can't cross the gap.

By increasing the emf of the supply you can find the pd at which no electrons are able to cross the gap. Even those with the maximum energy, Ekmax, can't do it.

At that point, the energy needed to cross the gap = maximum Ek of the electrons.

If you remember, work done moving a charge through a pd is W = QV and in this case charge, Q = e, the charge on an electron,

then

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Using this information you can calculate the maximum energy of the photoelectrons emitted from the metal.

Rewrite the photoelectric equation (from about 3 screens above this point) as

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or

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Plot a graph of the results and you get:

Photoelectric current and stopping Potential Diagrams

Obviously, nothing happens unless you get photons with energy above the threshold value, fo. After that, increasing the frequency of the photons increases the maximum kinetic energy of the photoelectrons so stopping them takes a larger value of stopping potential.

The gradient of this graph is Copyright S-cool . This is one method of calculating h, Planck's constant.

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